Abstract

Middle and Upper Jurassic strata in the Utah-Idaho-Wyoming thrust belt are the thickest, westernmost remnants of a north- to northwest-trending, broad shallow marine basin that formed east of a magmatic and orogenic highland. Those strata represent an early phase in the development of a major foreland basin. Jurassic strata are overlain by synorogenic coarse-grained detrital Cretaceous deposits.

Callovian and Oxfordian sandstones of the upper part of the Twin Creek Limestone (Giraffe Creek Member), the Preuss Redbeds and the Stump Formation have been studied at nine locations, covering a large part of the thrust belt. Modal analyses of 43 samples indicate compositional immaturity. Mono-crystalline quartz ranges from 40 to 83 percent of non-carbonate, non-opaque grains. Feldspars are more abundant than lithic grains and plagioclase exceeds potassium feldspar. Lithic grains are diverse with volcanic rock fragments predominating.

The sandstone compositions do not differ significantly from base to top. Composition differs between locations but spatial trends are subtle at best. Whereas most of the sand was probably derived from a magmatic arc and orogenic belt to the west, the paleogeographic setting suggests that some mature sand may have been derived from the north and southeast.